The present invention relates to a coordinate reading system for inputting a coordinate to an external device such as a computer.
As prior art coordinate reading systems, there exist Japanese Patent Laid-open No. 52/96825 and Japanese Patent Laid-open No. 55-96411, which were developed by the applicant of the present invention. Description will be made briefly about these coordinate reading systems.
FIG. 18 is a diagram illustrating a structure of the prior art coordinate reading system. A plurality of loop-shaped loop line groups 102 are laid on a tablet 101, a coordinate reading plate. These loop line groups 102 are selected one by one by a scanning circuit 107. An output s102 of the scanning circuit 107 is connected to a signal processing circuit 108 whose output is connected to a control circuit 109. The control circuit 109 also provides scanning signals s108 for selecting the loop line groups 102 sequentially in the scanning circuit 107.
A coil 113 is contained in a coordinate indicator 112 and is connected to an excitation signal generating circuit 111 contained in the main body of the coordinate reading system by an excitation signal cable 103.
In the coordinate reading system constructed as described above, coordinate values are calculated as follows. The coordinate indicator 112 is placed on the loop line groups 102 and always generates an AC magnetic field from the signal received from the excitation signal generating circuit 111. The control circuit 109 generates the scanning signal s108 and sequentially selects the scanning circuit 107. This operation is called "scanning." Since signals that correspond to a position where the coordinate indicator 112 is placed are induced in the loop line groups 102, the induced signal s102 for each loop line is sequentially inputted to the signal processing circuit 108 by selecting the loop line group 102 via the scanning circuit 107. The signal processing circuit 108 generates envelope waveforms of these induction signals. The control circuit 109 finds the coordinate values by inputting an amplitude of the induced signal for each loop line from this envelope waveform and by comparing and computing the amplitudes.
Accordingly, the prior art coordinate reading system is characterized in that it observes induced signals by scanning a large number of loop line groups.
In the prior art coordinate reading system, however, a large number of electronic switches such as analog switches had to be used for the scanning circuit to scan the whole tablet surface. This has been a factor which causes an increase in the cost. Moreover, there has been a problem that many loop lines have to be switched in order to detect the induced signals and the speed for detecting the position cannot be increased.